STEREO (Solar TErrestrial RElations Observatory) is a solar observation mission launched on 26 October 2006 at 00:52 GMT. Two nearly identical spacecraft were launched into orbits that cause them to pull respectively further ahead of and fall gradually behind the earth. This will enable stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.
The two STEREO spacecraft were launched at 0052 UTC on October 26, 2006 from Launch Pad 17B at the Cape Canaveral Air Force Station in Florida on a Delta II 7925-10L launcher into highly elliptical geocentric orbits. The apogee reached the Moon's orbit. On December 15, 2006, on the fifth orbit, the pair swung by the moon for a gravitational slingshot. Because the two spacecraft were in slightly different orbits, the "ahead" (A) spacecraft was ejected to a heliocentric orbit inside Earth's orbit while the "behind" (B) spacecraft remained temporarily in a high earth orbit. The B spacecraft encountered the Moon again on the same orbital revolution on January 21, 2007, ejecting it from earth orbit in the opposite direction from spacecraft A. Spacecraft B entered a heliocentric orbit outside the Earth's orbit. Spacecraft A will take 347 days to complete one revolution of the sun and Spacecraft B will take 387 days. The A spacecraft/sun/earth angle will increase at 21.650 deg/year. The B spacecraft/sun/earth angle will change -21.999 degrees per year. Their current locations are shown here.
Because the A spacecraft is moving faster than B, they are separating from each other and A is orbiting closer to the sun than B. This means stereoscopic pairs of images will soon be impossible for human eyes to fuse, which is a STEREO paradox. At the end of March 2007, the stereoscopic parallax was 1/50, but by June 2007 it was already 1/25. "Ideal" stereoscopic parallax is 1/30 and below 1/10 fusion is difficult even for experts. Already the east and west edges of the sun were becoming difficult, because one eye would see further around the sun than the other. Fortunately, the middle of the solar disc and up towards the poles will be fused stereoscopically after the edges become impossible. The A images, from the satellite closer to the sun, are bigger than B. Magnification must be corrected before stereoscopic fusion by human eyes is possible. Of course the mission does not depend on 3D vision to be useful and mathematical reduction of STEREO image data will continue.
Over time, the STEREO spacecraft will continue to separate from each other at a combined rate of approximately 44 degrees per year. There are no final positions for the spacecraft. They achieved 90 degrees separation on January 24, 2009, a condition known as quadrature. This is of interest because the mass ejections seen from the side on the limb by one spacecraft can potentially be observed by the in situ particle experiments of the other spacecraft. As they pass through Earth's Lagrangian points L4 and L5 (in late 2009), they will search for Lagrangian (trojan) asteroids. On February 6, 2011, the two spacecraft will be exactly 180 degrees apart from each other, allowing the entire Sun to be seen for the first time. Even as the STEREO spacecraft continue to separate, the addition of an Earth-based view, e.g. from the Solar Dynamics Observatory, will still provide full-Sun observations for several years. In 2015, contact will be lost for several months when the spacecraft pass behind the Sun. After this, they can continue to be operated after rolling by 180 degrees to point the high gain antenna at Earth. They will then start to approach Earth again, with closest approach sometime in 2023. They will not be recaptured into Earth orbit.
Science instrumentationA lunar transit of the sun captured during calibration of Stereo B's Ultra Violet imaging cameras. The Moon appears much smaller than it does seen from Earth, because the spacecraft-Moon separation was several times greater than the Earth-Moon distance.
Each of the spacecraft carries cameras, particle experiments and radio detectors in four instrument packages:
Launch weight including propellants was 1364 pounds (620 kg).
475 Watts from solar panels.
Data downlink: 720 kilobits per second.
STEREO's onboard computer systems are based on the Integrated Electronics Module (IEM), a device that combines core avionics in a single box. Each single-string spacecraft carries two 25 megahertz RAD6000 CPUs: one for Command/Data-handling, and one for Guidance-and-Control. Both are radiation hardened RAD6000 processors, based on IBM POWER CPUs (predecessor of the PowerPC chip found in older Macintoshes). The computers, slow by current personal computer standards, are typical for the radiation requirements needed on the STEREO mission.
For data storage, each spacecraft carries a solid state recorder able to store up to one gigabyte each. Its main processor collects and stores on the recorder images and other data from STEREO's instruments, which can then be sent back to Earth.
Published - July 2009
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